The invention relates to a method and an arrangement for judging the friction behavior between two counter-running partners, each of which has a contact surface, with the one contact surface moving along the other contact surface in the operating state.
There are numerous machines and systems in which two counter-running partners which are moved relative to one another for journalling, guiding or sealing off are provided which slide along one another during operation in order to perform the journalling, guiding or sealing function.
In internal combustion engines with reciprocating pistons for example the piston rings which are provided at the piston move along the working surface of the cylinder, which represents their counter-running partner. Furthermore, for sealing off, journalling and guiding the piston rod in reciprocating piston combustion engines, e.g. compressors, packings are usually used which are fixed with respect to the housing and which have a plurality of rings which slide on the piston rod as counter-running partner. Also, in bearings or slide ring sealings for journalling and sealing off rotating shafts, e.g. in pumps, two counter-running partners also move frictionally along one another. In this, one of the counter-running partners is rotationally fixedly connected to the rotating shaft and the other is rotationally fixed relative to the stationary part of the machine.
Common to all these examples is that the cooperation between the counter-running partners, which are moved relative to one another, is based on the friction between the counter-running partners. The counter-running partners can form either a dry-operated or a lubricated frictional system. In dry-operated systems, for example dry-operated piston compressors, no external lubrication is provided and a solid lubricant is usually embedded in at least one of the counter-running partners. In lubricated frictional systems, for example internal combustion engines, a fluid external lubricant, for example an oil or another liquid, is typically provided which forms a lubricating film between the counter-running partners.
The determination and monitoring by measurement of the friction between two counter-running partners during operation is as a rule possible only indirectly, for example via the arising wear. For large diesel engines, like the one used for example as drive aggregates for ships or in stationary plants for generating electricity, for example in EP-A-706 039 an inductive measurement principle is proposed by means of which the wear state of a specially manufactured piston ring can be determined by measurement. With measurements of this kind however only wear which has already arisen can be determined. It is however desirable to be able to judge the friction behavior between the counter-running partners during operation, since abnormalities or changes in the running behavior of the counter-running partners can be detected significantly earlier on the basis of changes in the friction behavior than on the basis of the wear which has already arisen. Severe and expensive damages, for example piston binding or abnormally high wear values, can thereby be avoided substantially more effectively.
An object of the invention is therefore to propose a method and an arrangement by means of which the friction behavior between two counter-running partners can be judged during operation and as rapidly as possible. The method and the arrangement should in particular be suitable for judging the friction behavior between the piston rings and the cylinder barrel in an internal combustion engine, especially in a large diesel engine. Furthermore, the method and the arrangement should also be suitable for counter-running partners which rotate relative to one another during operation, for example slide ring sealings.
The subjects of the invention satisfying these objects in regard to method and apparatus are characterized by the features of the independent claim of the respective category.
Thus in accordance with the invention a method and an arrangement are proposed for judging the friction behavior between two counter-running partners, each of which has a contact surface, with the one contact surface moving along the other contact surface in the operating state. The method in accordance with the invention comprises the following steps: one of the contact surfaces is charged by ultrasonic signals; ultrasonic signals which are reflected by the contact surface are received as echo signals; the echo signals are supplied to an evaluation unit as received signals; the Doppler shift of the received signals is determined by means of the evaluation unit; and the friction behavior between the counter-running partners is judged by means of the Doppler shift.
The arrangement in accordance with the invention comprises an ultrasonic converter and an evaluation unit, with the ultrasonic converter charging one of the contact surfaces with ultrasonic signals, receiving ultrasonic signals which are reflected from the contact surface as echo signals and supplying the latter to the evaluation unit as received signals, and with the evaluation unit comprising evaluation means which determines the Doppler shift of the received signals and determines a characteristic value for judging the friction behavior between the counter-running partners by means of the Doppler shift.
The invention is based on the observation that the friction between two counter-running partners causes a Doppler shift in ultrasonic signals with which the contact surface of a counter-running partner is charged which is sufficiently strongly pronounced in order to enable a judging of the friction behavior. Changes in the friction in particular lead to significant changes in the spectrum of the Doppler shift. Thus if an increase in the friction between the counter-running partners arises during operation, for example as a result of a faulty lubrication, or a reduction of the friction, for example through a damaged or worn piston ring, then this can be detected on the basis of the Doppler shift and corresponding countermeasures can be taken in time in order to avoid severe resultant damages.
Since the ultrasonic converter must have no direct physical contact with the contact surface the method in accordance with the invention and the arrangement in accordance with the invention are also suitable for judging the friction behavior at locations which are inaccessible or are difficult of access, e.g. for judging the friction behavior between a piston ring and the cylinder barrel in an internal combustion engine. In addition the method and the arrangement respectively can be used without problem during the normal operating state of a machine, so that a judging and thus a monitoring of the momentary friction behavior between the counter-running partners are possible as if it were on-line.
It turned out that an increase in the friction between the counter-running partners leads in particular to an amplification or to an increase respectively of those components of the Doppler shift which lie at high frequencies. Therefore, preferably only those frequencies in the frequency spectrum for the Doppler shift which are greater than a predetermined threshold value are taken into account for the judging of the friction behavior.
For reasons of as simple an evaluation as possible it is preferred when contact surface is charged with the ultrasonic signals substantially perpendicularly to the direction of the relative movement of the two counter-running partners.
In accordance with a first exemplary embodiment of the invention the contact surface is charged with ultrasonic pulses, which means that the ultrasonic converter emits wave packets as ultrasonic pulses at regular intervals.
In accordance with a second exemplary embodiment the contact surface is charged by a continuous ultrasonic wave, which means that in this method, which is usually called the CW (continuous wave) method, the ultrasonic converter does not emit the ultrasonic signals in the form of individual discrete pulses, but rather permanently as a continuous wave. In the CW method the ultrasonic converter preferably comprises two converter elements, one of which serves for emitting the ultrasonic signal and the other for receiving the echo signals. The two converter elements are arranged in such a manner that the emitter field of the first converter element and the reception field of the second converter element at least partly overlap. This can be realized for example in that the two converter elements are arranged adjacently and where appropriate with a slight inclination with respect to one another, so that their axes intersect at the contact surface. The two converter elements can however also be arranged concentrically.
A preferred use of the method in accordance with the invention or of the arrangement in accordance with the invention is the use for judging the friction behavior between a piston ring and its counter-running partner in an internal combustion engine, in particular in a large diesel engine, e.g. in the main engine of a ship.
Further advantageous measures and preferred embodiments of the invention result from the subordinate claims.